One of the more difficult pollution control problems is the combustion of an organic substrate comprised of one or more organic compounds and, in particular, the treatment of an organic substrate contained in a dilute gaseous stream. Such streams are, for example, automotive spray booth emissions, vent streams from industrial processes, storage bin vent emissions and vent streams from coil and can coating processes. If the stream is exceptionally dilute (such as on the order of ten parts per million or less by volume), the organic substrate can be removed, ordinarily, by passage of the stream through a packed bed of carbon adsorbent. If, however, the organic substrate in the stream to be treated is prone to polymerize, the carbon bed will not be regenerable, i.e. reusable, and must frequently be replaced at both considerable cost and inconvenience. If the stream is moderately dilute (such as on the order of ten to one thousand parts per million by volume), adsorption on carbon is impractical. Under such circumstances, the carbon bed becomes saturated so quickly that it must be regenerated at impractically and even impossibly frequent intervals. Polymerizable substrates compound the problem by making regeneration that much more difficult.
The alternative to adsorption is incineration. In accordance with this method, instead of simply removing the organic substrate, it is converted by oxidation (i.e. combustion) in an incinerator to essentially carbon dioxide and water. Such end products, in the low total concentration found in the effluent, are innocuous, and hence pollution is abated. There are two types of incineration, commonly referred to as "Steady State" combustion, which may be employed, namely thermal and catalytic. Both are employed for pollution control, i.e. removal of an organic substrate from a dilute stream, with various designs and modifications being available. Both of these methods rely on elevated temperatures to cause the oxidative degradation of the organic substrate.
In thermal incineration the dilute stream (containing at least a few percent of a stoichiometric excess of oxygen for the combustion of the substrate to essentially carbon dioxide and water) is heated to a high temperature, of the order of 700.degree. C., for a period of about one second. Treatment in this fashion generally can reduce the content of organic substrate in the stream by some 80 to 90 percent. The higher the temperature to which the dilute stream is heated, the greater the diminution of the organic substrate which can be achieved. Alternatively, in theory, the residence time may be increased to achieve greater combustion, but this is generally found to be impractical. By going to exceedingly high temperatures of combustion (such as on the order of 1000.degree. C.) 99.9 percent or greater combustion can usually be achieved.
In practice, abatement of pollution by thermal incineration of the organic substrate has several disadvantages, especially when the substrate is found in a dilute gaseous stream. For example, it is usually necessary to use an auxiliary fuel, such as methane, propane and other combustible hydrocarbons. Indeed, safety requirements may preclude thermally incinerating a stream which is so rich in the organic substrate as to sustain combustion without an auxiliary fuel. In the case of a dilute or moderately dilute stream, nearly all the heat necessary to reach the combustion temperature (on the order of 600.degree. C. to 900.degree. C.) comes from the auxiliary fuel. If the heat from incineration can be recovered and is of value, the cost of the auxiliary fuel is partly offset by the value of the recovered heat. However, the recovery of the heat from the incineration requires a large capital investment, and invariably the amount of heat recovered is less than that initially provided by the auxiliary fuel. On the other hand, if there is no value to the recovered heat, the auxiliary fuel is essentially wasted, serving only to heat the stream to the temperature at which combustion of the organic substrate occurs. Furthermore, where greater than a 90 percent reduction in the organic content of the stream is desired, i.e. over 90% combustion of the organic substrate, the thermal conditions become exceedingly severe, and more expensive materials of construction are required and more frequent failure of the equipment occurs.
The aforementioned catalytic incineration method uses a combustion catalyst to facilitate the combustion reaction. With a catalyst present, combustion generally proceeds at only moderately high temperatures (such as on the order of 300.degree.-400.degree. C.) with a residence time on the order of 0.1 second. The extent of conversion is again 80 to 90 percent, with a higher reduction in the organic level being attainable by operating at higher temperatures or by increasing the residence time of the organic substrate with the combustion catalyst.
Catalytic incineration also has several disadvantages. Although the temperature to which the dilute stream must be raised is less than in thermal incineration, the use of an auxiliary fuel is still required unless the concentration of the organic substrate is moderately high (such as on the order of 5000 parts per million by volume). As in thermal incineration, if the heat produced is not recovered, the value of the auxiliary fuel is lost. Improved performance, i.e. greater reduction in the amount of organic substrate, is again achieved by raising the operating temperature but this invariably requires the use of a large quantity of auxiliary fuel. Furthermore, the catalysts used in catalytic incineration tend to "age" and thus performance deteriorates with time. This deterioration is accelerated at high combustion temperatures. In addition, the combustion catalyst may be poisoned by the inadvertent catalytic incineration of certain materials, e.g. sulfur-containing compounds, resulting in catalyst deactivation. If the catalyst is contacted below the temperature of combustion with an organic substrate which tends to polymerize at a temperature below its temperature of combustion, there can be formed a layer of coke, or a polymer film can be deposited on the catalyst surface. This could inhibit further combustion of the substrate unless very high temperatures, sufficient to remove the deposit, are employed. It follows that deactivation can necessitate catalyst replacement, a costly and time consuming procedure.
From the foregoing it is evident that treatment of an organic substrate in a dilute stream presents a difficult pollution abatement problem. Heretofore, such treatment required a large capital investment (in adsorption beds or incineration units) and usually resulted in high operating costs (auxiliary fuel, combustion catalyst replacement, or carbon bed replacement).
The various problems outlined above have been recognized by workers in the field for many years. One possible solution to these various problems is to combine adsorption and combustion. In this method an adsorbent is used to concentrate the organic substrate and a catalyst is used to combust the adsorbed organic substrate after it is desorbed from the adsorbent. In this fashion, the use of auxiliary fuel is restricted to that period during which desorption/combustion occurs, and the use of the combustion catalyst minimizes the ultimate temperature required to achieve combustion.
The method of combining adsorption and combustion is disclosed in U.S. Pat. No. 3,658,724, to Stiles, although the disclosed adsorbent/oxidation catalyst suffers from several inherent disadvantages. As disclosed in Stiles, the porous adsorbent shape of the catalyst of Stiles is made by mixing "activated carbon particles with a gel forming material". Although the use of carbon as an adsorbent is often desirable, in that carbon has a high adsorption capacity, it possesses poor oxidation stability. In fact, at temperatures as low as 300.degree. C. carbon can begin to combust, as is shown in "Catalytic Oxidation of Vapors Adsorbed on Activated Carbon", Environmental Science and Technology, 9,846 (1975). This poor oxidation stability for carbon adsorbents is a major disadvantage, since at the higher temperatures at which the organic substrate is combusted carbon itself may undergo combustion. As a result, carbon can usually only be used to treat very dilute streams at ambient or slightly above ambient temperatures. Furthermore, the adsorptive capacity of carbon decreases at elevated temperatures. Consequently, desorption is rapid at elevated temperatures, and as combustion begins the consequent heat release causes a large quantity of organic substrate to be suddenly released. Unless combustion on the catalyst is extremely efficient, which usually is not the case, or the catalyst bed is very deep there will be emission of the organic substrate into the effluent stream and subsequently out of the system as a pollutant.
To overcome the problems associated with using carbon as the adsorbent U.S. Pat. No. 3,658,724 discloses that other adsorbent materials, other than carbon, may be used. Among the materials disclosed are silica, alumina or various metal oxides. Although these materials have enhanced thermal stability they also have relatively low adsorptive capacities and consequently the organic substrate is only very weakly bound to them. Because of this weak binding, when combustion or heating occurs the organic substrate tends to be readily desorbed and passes into the effluent stream uncombusted. In addition, the low adsorptive capacities of these materials preclude the accumulation of sufficient organic substrate to generate a large temperature rise in the catalyst bed. As a result, the organic substrate is not heated sufficiently and incomplete combustion of the organic substrate occurs.
In addition to the above adsorbent materials, U.S. Pat. No. 3,658,724, to Stiles, discloses that molecular sieves may be used as the adsorbent with a combustion catalyst. This general reference to molecular sieves is inadequate, in several respects, in solving the many problems associated with the combustion of an organic substrate in a dilute stream. Firstly, the term "molecular sieve" refers to a wide variety of materials, many of which are unsuitable for an adsorption/combustion process. For example, some molecular sieves are amorphous and thus lack the crystalline structure necessary for use in an efficient adsorption/combustion process. In addition, one class of molecular sieves are the zeolites. However, zeolites are not in general sufficiently thermally stable to withstand the temperatures that would be expected to occur during an adsorption/combustion process. In fact, they often lose their crystallinity at less than 600.degree. C. Ordinary zeolites tend also to strongly adsorb water, a product of combustion, and only very weakly adsorb organics, thus making them unsuitable for an efficient adsorption/combustion process.
Stiles, at column 2, lines 14 et seq., discloses the use of an adsorbent by first adsorbing the organic substrate on the adsorbent and then after a substantial amount has been adsorbed, the substrate is driven from the adsorbent by heating. As the organic substrate in this case leaves the adsorbent, at least a portion of the substrate is expected to contact an oxidation catalyst incorporated within and on the surface of the adsorbent. Thus, the ultimate process results in some of the organic substrate desorbing uncombusted into the dilute stream since the substrate may not necessarily have contacted the oxidation catalyst after it leaves the adsorbent and consequently the substrate passes uncombusted into the dilute effluent.
The use of high-silica zeolites for the combustion of an organic substrate is disclosed in co-pending U.S. patent application Ser. No. 864,835, filed Dec. 27, 1977. This application discloses a "Steady State" combustion process for the combustion of an organic substrate to essentially carbon dioxide and water. The disclosed process oxidatively combusts the organic substrate by contacting the preheated organic substrate with a catalyst bed comprising a high-silica zeolite. When the preheated organic substrate contacts the catalyst bed a "Steady State" combustion of the organic substrate takes place. The process requires that the organic substrate be preheated to a relatively high minimum temperature before it contacts the catalyst bed in order that a high percentage of the organic substrate be combusted to essentially carbon dioxide and water, as discussed in the context of example 3-9 hereof.
The present invention overcomes the difficulties of the prior art by utilizing the unique properties of high-silica zeolites in an adsorption and combustion process. It has been found that the high-silica zeolites have an unusually strong adsorption affinity for organic compounds, i.e. organophilicity. The very high organophilicity and, in addition, the hydrophobicity of these zeolites make them ideally suited for use in the oxidative combustion of an organic substrate. In addition, the thermal and hydrothermal stability of these zeolites make them well suited for combustion of even the most refractory organic substrate.
It has been discovered that these high-silica zeolites, when containing a suitable combustion catalyst, may be used in an adsorption/combustion process involving the adsorption of the organic and subsequent combustion of the organic substrate with the metal-containing zeolite.
Thus, the present process, as distinguished from the prior art, provides a process wherein a high-silica zeolite, containing a suitable combustion catalyst such as Pt, Pd, Cu, Ni, Cr, and Mn, adsorbs and combusts an organic substrate without the necessity of preheating the organic substrate to a high temperature prior to contact with the metal containing high-silica zeolite and, further without desorption of uncombusted substrate into the effluent. These advantages and other will be more fully discussed hereinafter.